Why is fog in polluted areas often more acidic than rain?

In polluted regions, fog and cloud water are often significantly more acidic than rain falling over the same area. This difference occurs because fog droplets are much smaller than raindrops, which leads to a higher concentration of acidic pollutants within the water.1

While rain droplets are large and fall quickly through the atmosphere, fog consists of tiny suspended droplets that remain in the air for longer periods. These droplets provide a concentrated medium for the aqueous-phase reactions that convert sulfur dioxide ($S{O}_2$) and nitrogen oxides ($N{O}_x$) into sulfuric acid and nitric acid. Because fog droplets have a much smaller volume of water to dilute these acids compared to the larger volume of a raindrop, the resulting concentration of hydrogen ions ($H^{+}$)—and thus the acidity—is substantially higher.12